The field of motor and generator design comprises synchronous and induction motors and generators. In such motors and generators, a rotating, or more generally speaking a traveling, magnetic field is generated with the aid of inverters. The inverters have switches, such as IGBT's (insulated-gate bipolar transistors) or MOSFETs (metal-oxide-semiconductor field-effect transistors) that control currents through wires that are wound around a magnetic core. In some cases, the MOSFETs are controlled by a microprocessor that generates the appropriate currents at an appropriate frequency in each of the windings. In some such cases, the MOSFETs use pulse width modulation (PWM) signals to control the current through the MOSFETs, and so through the associated windings.
Windings (typically in the stator) are a necessary component of current induction motors and generators. In motors the windings are excited with electrical current to generate a magnetic field that interacts with a magnetic field associated with a rotor. The interaction of the magnetic field generated in the stator with the magnetic field in the rotor causes the rotor to move with respect to the stator. In a generator relative motion of a rotor with respect to a stator causes electrical current to be generated within the windings of the stator as a consequence of the motion of the magnetic field of the rotor with respect to the stationary windings on the stator.
There are several limitations and costs to having windings in a motor or generator. These are well known to those skilled in the art and are not enumerated here for the sake of brevity. In light of these limitations, it would be desirable to provide an induction motor or generator in which a controllable electro-magnetic field can be generated in the rotor without the need for windings.
In addition, having a motor or generator in which the rotor and stator are in very close proximity provides a more efficient motor or generator. In current motors and generators, mechanical bearings are used to maintain a magnetic flux air gap between the moving rotor and the stationary stator. The higher the quality of the bearings, the more efficient the motor or generator. However, high quality bearings are expensive and over time, can fail due to wear that occurs as a result of the relative motion of the rotor and stator.
Accordingly, there is presently a desire for a motor/generator design that does not require windings wound around a core and that has a very small magnetic flux air gap between the stator and rotor with less reliance on bearings to maintain the gap.